Rheological characterization of low-density polyethylene in planar extension using rheo-optics

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Abstract
Optical techniques such as birefringence and Laser Doppler velocimetry can be used to characterize the flow behavior of polymeric liquids in complex flow geometries. This use of optical techniques is called rheo-optics. In the study reported here, rheo-optical measurements were carried out on a low density polyethylene (LDPE) melt in a converging rectangular slit die. Using Laser Doppler velocity measurements as input data, extensional stresses along the center line of the converging slit were calculated using two different constitutive equations, the Wagner equation and the Phan-Thien and Tanner (PTT) equation. The calculated curves were compared with experimental stress data as obtained from birefringence experiments. It was found that calculations based on the PTT model give a better agreement with the experimental data than Wagner's equation. The application of rheo-optical measurements has the great advantage that local information is obtained on both the stress field and the velocity field in non-homogeneous flows. However, a number of possible causes of errors are introduced that should carefully be considered. Some of the problems involved in rheo-optical experiments are discussed.

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